Bearing shells for journaled engine bearings typically comprise a strong steel backing layer, a lining layer, and optional diffusion barrier layer, and an overlay layer that provides the running surface for the journaled shaft, e.g. a hollow generally semi-cylindrical steel backing layer, a copper-based alloy lining layer, and a tin or tin-based alloy overlay on the inner surface. A generally semi-annular thrust flange is provided at each axial end of the bearing shell before being assembled into the bearing of an engine.
It is desirable to provide increased wear resistance and to improve the fatigue strength of layers in bearing linings, particularly overlay layers. A particular challenge to bearing lining layer and/or overlay layer performance is provided by the configuration of vehicle engines to save fuel by using a stop-start operation, in which the engine is stopped each time the vehicle stops, in contrast to conventional engine operation, in which the engine is kept running throughout a vehicle's journey. Engines configured for stop-start operation may restart their engines about one hundred times more frequently than conventionally configured engines run continuously throughout each vehicle journey. The particular problem that an engine configured for stop-start operation presents arises because engine bearings are conventionally hydrodynamically lubricated, with little or no lubrication initially being provided to the bearings when the engine starts, leading to particularly significant wear during the start-up phase.
It has been proposed to increase the wear resistance of engine bearings by the incorporation of hard inorganic micro-particles, which are substantially insoluble in the electroplating electrolyte, into bearing linings. Exemplary materials are the incorporation of aluminium oxide, silicon nitride, silicon carbide or boron carbide hard particulate into a tin-based alloy matrix. However, the production of composite layers with hard particulate in high enough concentrations to be useful is difficult by conventional electroplating techniques, particularly in a tin-based alloy matrix (e.g. at least 50% wt tin), and most particularly in the case of a pure tin matrix (e.g. ≧50% wt tin). Further, such materials commonly suffer from low tensile strength and ductability, as well as particle cracking.